Accelerate the arm to keep hands in front of the body
Using an inside-horse versus outside-horse image, the coach points out that the arms travel a longer path than the body core, so they cannot arrive at the ball together if both move at the same speed. He says that is why arm speed relative to body speed sometimes has to be greater, and that especially on a longer swing you need to get the left arm accelerating to keep the hands in front of the body.
Channel: PorzakGolf
Watch official source at 7:38